Constant pressure systems



CONSTANT PRESSURE SYSTEMS Bernhardt Joseph Eiseman, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware N Drawing. Application July 18, 1955 Serial No. 522,877

4 Claims. (Cl. 252-62) matic installation actuated by pressure changes, a constant pressure system would be incorporated in a Wheatstone bridge type of electronic control circuit. Another application of a constant pressure system is to be found in a fixed-volume system where temperature changes and resultant difference in inside and outside pressure would normally cause deformation of the physical materials comprising the periphery of the system.

The latter type application of a constant pressure systern, in a fixed volume, is exemplified in present day re-. frigerators and cold storage boxes. It is known that the use of fibrous insulation between the interior and exterior walls of a refrigerator for all practical purposes eliminates convection and radiation losses; however, a factor which causes inefiiciency of said insulation system is gaseous conduction. At the present time, the gaseous material between the walls of the refrigerator is air. It is known, however, that gaseous conduction is inversely proportional to the square root of the molecular weight of the gas (other things being equal and the mean free path being small compared to the interstices) and it becomes apparent that by use of gases heavier than air a more efiicient insulating system would result.

In order to more advantageously use a gas between the interior and exterior walls of a refrigerator, the gas must not only have a molecular weight higher than that of air, but said gas should also be non-flammable, nontoxic, relatively inexpensive, and available commercially. Fluorinated hydrocarbon gases have an obvious advantage over air for such use since they have the character istics just enumerated.

It is an object of this invention to provide a process for maintaining a constant pressure system which will effect efficient insulation for refrigeration and thermal'insulation panels. It is a further object of this invention to reduce the change of pressure over a range of temperatures in a closed fixed-volume system thus increasing the insulating capacity of said system in addition to making possible the use of economical design and lighter construction materials.

In the present day refrigerators, the air-filled insulation is vented to maintain the air at the ambient pressure. When using fluorinated hydrocarbon gases, it is necessary to utilize a sealed system; however, in such a system differences in pressure between said system and the ambient air will develop on heating or cooling. in the ordinary refrigerator and freezer, inside temperatures will range from 40 C. during storage to 40 C. during use which temperature dii'ierences may cause as much as 5 lbs. per square inch pressure variation and can result in excessive buckling or other damage to the thin metal wall of the ited Sates ae 'O 2,865,860 Patented Dec. 23, 1958 refrigeration panels. This buckling and deformation could be prevented by re-inforcing the walls of the system, but such a technique would increase the cost and also decrease insulating efficiency because of increased heat conduction. The closed systems just described are useful not only for refrigerator systems but also for thermal insulation panels generally. The use of a fluorinated alkane gas to reduce gaseous conduction in such panels has the added advantage of enabling the use of thinner panels to result in more efiicient thermal insulation.

The process of the present invention for maintaining constant pressure systems which will avoid the problems above-discussed, is accomplished by maintaining in a t closed fixed-volume system a gaseous phase and aliquid may be straight chained or cyclic and may contain halogens other than fluorine. The selection of the particular fluoroalkane to be used will depend upon the temperature range to be encountered and the pressure over Which the particular system is to operate. In addition, the fiuoroalkane will be selected in conjunction with the crystallizable organic solid to be used in the system. Furthermore, the selected gaseous fluorinated alkane must have a solubility,'in its mutual solution with the organic solid, which will not exceed about 10 mole percent. If this solubility limit is greatly exceeded, an inoperable system Will result.

Examples of fluorinated alkanes which will be of value as the gaseous phase of the constant pressure system are: fluorotrichloromethane, dichlorodifluoromethane, chlorotrifluoromethane, tetrafluoromethane, dichlorofluoromethane, chlo-rodifluoromethane, trifiuoromethane, chlorofluoromethane, chloropentafluoroethane, hexafiuoroethane, pentafluoroethane, bromochlorodifluoromethane, bromotrifiuoromethane, bromodifluoromethane, bromopentafiuoroethane, perfiuorocyclobutane and the like.

The solid which is incorporated with the fluoroalkane in the fixed-volume system may be selected from a wide variety of organic compounds providing these compounds meet the following qualifications:

1) They must be mutually soluble with the fiuorinated alkane gas to form a liquid solution.

(2) They must crystallize out of their mutual solution with the fluoroalkane.

(3) They must not chemically react with the gaseous component.

By solid, it is meant that the substance is a solid under conditions of use. Nitrobenzene for example, normally a liquid, melts at 5.6 C., but is an operable solid in systems which function at .temperatures somewhat above 6 C. and below. As discussed later, the system can be operated at temperatures somewhat above the melting point of the solid in order to extend the effective temperature range.

Preferably the crystallizable solids will be members of the class of halogenated aliphatic hydrocarbons, aromatic hydrocarbons, and halogenated aromatic hydrocarbons. Specific examples of these preferred classes include the fluorine-containing halogenated alkanes such as 1,1-difluorotetrachloroethane, and 1,2-difluorotetrachloroethane; aromatic hydrocarbons such as benzene, naphthalene, beta-methylnaphthalene; halogenated aromatic hydrocarbons such as p-dichlorobenzene, o-chlorobiphenyl, beta-chloronaphthalene, and p-bromotoluene; less preferably, nitrated aromatic hydrocarbons such as nitrobenzene, alpha-nitronaphthalene, and meta-nitrotoluene.

' 'As with the selection of the fiuorinated alkane, the crystallizable organic solid will be chosen on the basis of 3 the temperature range'over which the fixed-volume system is to be used. Normally a solid is selected that will have a melting point about 5 to C. lower than the maximum temperature of the range over which the system percent under the conditions of use. The system is establish d by charging it with the organic solid and then introducing the gaseous fiuorinated alkane to the desired, pressure. Preferably, no more than about 10 percentof the total volume will be occupied by the condensed phase. Systems in which the liquid phase occupies more than 10 per cent of the total volume of the system are operable and useful, but it is desirable that the liquid phase' occupy the smallest possible volume.

The operable temperature range may be extended by I using mixtures of crystallizable organic solids and/or mixtures of gases. An additional technique for tem erature range extension is to use mixtures of solids whi h a give partrlly miscible or immiscible liquid phases with their mutual solution of the gas.

Performance is best when equilibrium is maintained so that the mutual liquid solution of gas and solid is saturated with respect to the solid as well as the gas. That is. there will normally be three phases within the fixedvolume system in which the pressure is to remain essentiallv constant.

There will be a vapor phase due to the fluorine-containin alkane itself, a solid phase comp ising the solid crystallizable or anic compound and a liquid phase comprising the liquid mutual solution of gaseous al ane and solid organic compound. The temperature range over which the system normally operates can be extended somewhat by going beyond the re ion in which th ee phases are coexistent, but this can be done on y as long as tolerable pressure changes are not exceeded.

The process of this invention functions by virtue of the fact that the mutual solution of fiuorinated alkane and crystallizable organic compound serves as a bufier against pressure changes due to temperature efiects. As

I the temperature decreases, the pressure of a fixed-volume gaseous system would normally decrease.

However, in the system described herein, a temperature decrease also causes the organic compound to crystallize out of its mutual solution and as a result, the fluorinated alkane is evolved as vapor, its increased amount thus compensating for the pressure drop due to the temperature decrease.

. Conversely, when the temperature rises, the solid organic compounds melts, thus decreasing the concentration of gas in the mutual solution and allowing more gas to dissolve, which in turn ofisets the expected pressure increase due to the temperature rise.

For the present system to be operable, the solubility of the fiuorinated alkane in the crystallizable organic solid should be less than 10 mole percent. If this solubility limit is exceeded, the system fails to function properly; this failure is due to the fact that the solubility of the gas in its mutual solution is so great that the gas is not evolved'when the liquid is concentrated by crystallizing out solid on temperature reduction. Since gas is i not evolved, the pressure falls and the system is no longer [constant pressure. p .The gaseous alkane must have some minimum solubility with the solid in order for the system to function. lower limit willdepend on the particular choice. of

aseaseo components, but will be within the range of 0.5 to 1 mole percent. i I In the present invention it is desirable that the liquid phase occupy the smallest possible volume. It has been observed that in the process of this invention, the liquid phase will normally occupy less than 10 percent of the total volume of the system. In insulated panels, for example, the liquid phase may be absorbed through the fibrous ceramic materials (such as roc or mineral or glass wool) used inside said panels; this will provide a large surface area and will add in maintaining equilibrium conditions. The condensed phase will distill or sublime onto the colder wall and form a layer over the large area which will serve to prevent conduction through the insulation by the condensed phase of the system.

The process of this invention will normally enable a fixed-volume system, as described, to maintain a pressure I not varying more than about 10 percent over the range of temperatures desired. Also significant, however, is the fact that the novel systems of this invention permit control of pressure changes even over rather small temperature ranges. In this latter instance, the expected pressure change due to a change in temperature may be within 10% of the original pressure, but by use of the process of this invention the actual pressure change may be reduced to a small fraction of the expected change. Where large easily deformed surfaces are exposed to even small pressure changes, this control will be particularly valuable, since even small pressure changes can greatly affect large unsupported areas. The process is not limited to any particular pressure and may be applied to subatmos pheric as well as superatmospheric pressures, although in most practical applications, the process will be used to maintain an essentially constant pressure which falls within 10 to lbs/sq. in. absolute. Preferably, the process will be used at about 10 to 20 lbs/sq. in. absolute; i. e., at about atmospheric pressure where it will find utility in insulation systems.

EXAMPLE 1 into the tube as the crystallizable organic solid. The top was replaced, the tube immersed in liquid nitrogen and the valve at the top closure connected to a vacuum pump. When the pressure inside the system was reduced to 0.01 mm. Hg, the valve was closed, and the pump removed. The tube was then removed from the liquid nitrogen bath and warmed to 32 C. (rm. temp.) by immersing it up to the mark in a water bath. Bromotrifiuoromethaue gas was then introduced through the valve at 32 C. until saturation was reached at 2.5 p. s. i. g. As the gas was added, the solid phase inside the tube decreased and liquid phase appeared. When 2.5 p. s. i. g. was reached, about 7 mole percent of the gas was in solution and the condensed phase occupied about 6% of the volume up to the mark and only a trace of solid remained.

The system was shown to maintain an essentially constant pressure by varying the temperature of the water bath and noting, by means of the gauge, the small change in pressure that actually occurred. The table which follows indicates the small pressure change with varying P 1 in? i asses-so Calculations by the gas laws show that the temperature decrease would be expected to reduce the pressure of the gsystem from 16.7 p. s. i. a. at 35.6 C. to 13.9 p. s. i. a. EXAMPLEZ Following the details of Example "'1, the system was charged with 4.8 parts of beta-methylnaphthalene and dichlorodifluoromethane gas was added to a saturation pressure of 2.8 p. s. i. g. at 29 C. There was about 7 mole percent of gas in solution and the condensed phase :of the system occupied 3.2% of the volume up to the mark. The table which follows indicates the slight .change in pressure over the temperature range of 35 C.

. c V {0 15 C. Table Temperature Pressurein of system, P. S. I. A

Table III Bath tem- Pressure peratire, of system, C. P. S. I. A.

6 EXAMPLE 4 As in Example 1, the tube was chargedwith 10.6' grams of 1,2-difluorotetrachloroethane and chlorotr'ifiuoromethane charged into the system to a saturation pressure of 17 p. s. i. a. at 24.2 C. There was about 3 mole percent of gas in solution and the condensed phase occupied about 4% of the volume up to the mark. The following table illustrates the effectiveness of the system over a temperature range:

The table which follows describes several additional systems which will maintain a pressure of about 15 p. s. i. a. over the range of temperatures indicated:

Following the procedure 'of Example 1, 39.2 'parts of beta-methylnaphthalene are introduced in a fixedvolume system and after evacuation is pressured with a mixture of gases comprising 3.1 parts of CClzFg and 0.5 part by weight of CBrF Approximately 10% of the total volume of the system is occupied by the condenced phase and about 4 mole percent of the gases is in solution at 1 atmosphere and at 35 C. This system will maintain a pressure of essentially 15 p. s. i. a. over a temperature range of from +35 to 10" C.

EXAMPLE 7 A mixture of crystallizable organic solids can be employed as follows:

A mixture of 23 parts of naphthalene and 40 parts of p-dichlorobenzene (melting point of solid mixture=29 C.) is placed in'a constant volume system and after evacuation the system pressured with 1.1 parts by weight of CBrF About 1.2 mole percent of the gas dissolves at 1 atmosphere pressure and at 35 C. and the condensed phase occupies about 9% of the total volume of the system. When the system is subjected to tempera tures between 35 and 10 C., the pressure of the system remains essentially at 15 p. s. i. 3..

EXAMPLE 8 The operating range of the system described in Example 7 can be extended to higher temperatures by' organiccompounds so :7 adjusting the compositionof thelimixture of crystallizable that it has a higher melting point.

Byfichargi'ng; he: system.w1th a mixture comprising 11.4

.parts of. naphthalene and 51.4 parts of p-dichloroben- .zene (melting point of mixture=4l C.) and pressuring 1 theevacuated system with 0.8. part of CBrF so that 0.8 mole percent of the gas is in solution and the condensed 1 phase occupies 10% of the total volume, the system maintained a pressure of essentially 15 p. s. i. a. o ver a temperature range of 50 to 10 C.

EXAMPLE 9 19% of the total volume, a system iS obtained which maintains an essentially constant pressure of 15 p. s. i. a. over temperatures from +45 C. to C.

EXAMPLE 3 The beta-methylnaphthalene of Example 9 is replaced with 18 parts of nitrobenzene. The system is pressured with CBrF at +20 C. and p. s. i. a. until 0.7 mole percent is in solution andthe two immiscible liquid ,phases obtained occupy about 22% of the total volume. In this way a system is obtained which will maintain an essentially constant pressure of 15 p. s. i. a. over the temperature range of to 40 C.

Another application of thefconstant pressure systems .of this invention is in those instances where volume changes have-a thermal efiect such as in a liquid spray system wherein a liquefied gas vaporizes and thus expands to displace a diaphragm or piston which in turn forces liquid through a spray nozzle,,said system being activated ;by a valve which opens the system to the atmospherei When open to the atmosphere, the liquefied gas vaporizes and a temperature drop results; said drop in temperature reduces the vapor pressure of the liquefied gas and thus reduces the pressure acting upon the diaphragm or piston. If constant pressure is maintained in spite of this thermal efiect, a more efficient system would result.

This invention provides a constant pressure system in such applications, since by its use the thermal efiect of vaporization or condensationbf the gas is offset. This results from the factthat although the temperature tends to drop on the vaporization of the liquefied gas, the solid crystallizes out which-tends to raise the temperature because of the heat of crystallization of the solid. The latter offsets the cooling effect and the pressure remains essentially constant. Components of high purity should be used since any impurities present mayaccumulate in the liquid phase andlower the vapor pressure. Solids which readily form supersaturated solutions should be avoided.

In these systems, constant pressure with varying ambient temperature, as above described, is also desirable.

I claim:

l. A process for reducing changes in pressure, said pressure being within the range of 10 to 100 p. s. i. a. in a closed essentially fixed-volume system subjected to vary ing temperatures within the range of 90 C. to 40 C., said temperatures within 'said range varying from about C. below the melting point or the crystallizable organic solid to about 10 C. above said melting point, which process comprises maintaining in said system a gaseous fluorine-containing halogenated lower alkaue of from 1 to 4 carbon atoms and a liquid saturated equilibrium mutual solution of a crystallizable organic solid taken from the group consisting of halogenated aliphatic hydrocarbons, 1,4-butanediol, aromatic hydrocarbons, halogenated aromatic hydrocarbons, and nitrated aromatic hydrocarbons, and, said fluorine-containing halogenated alkane, said fluorine-containing halogenated alkane being present in said mutual solution in the amount of about 0.5 to about 10 mole percent, said crystallizable organic solid being mutually soluble with and chemically inert to said gaseous fluorinated alkane, and, said organic solid being crystallizable out of said mutual solution.

2. The process of claim 1 in which the gaseous fluorinecontaining halogenated alkane is a gaseous mixture of said fluorine-containing halogenated alkanes.

3. The process of claim 1 in which the crystallizable organic solid is a mixture; of said crystallizable organic solids.

4. The process of claim 1 wherein about 10% of the total solution ofsaid system is occupied by said mutual solution.

References Cited in the file of this patent UNITED STATES PATENTS 2,070,167 Iddings "a Feb. 9, 1937 

1. A PROCESS FOR REDUCING CHANGES IN PRESSURE, SAID PRESSURE BEING WITHIN THE RANGE OF 10 TO 100 P. S. I. A. IN A CLOSED ESSENTIALLY FIXED-VOLUME SYSTEM SUBJECTED TO VARYING TEMPERATURES WITHIN THE RANGE OF 90* C. TO -40* C., SAID TEMPERATURES WITHIN SAID RANGE VARYING FROM ABOUT 50*C. BELOW THE MELTING POINT OF THE CRYSTALLIZABLE ORGANIC SOLID TO ABOUT 10*C. ABOVE SAID MELTING POINT, WHICH PROCESS COMPRISES MAINTAINING IN SAID SYSTEM A GASEOUS FLUORINE-CONTAINING HALOGENATED LOWER ALKANE OF FROM 1 TO 4 CARBON ATOMS AND A LIQUID SATURATED EQUILIBRIUM MUTUAL SOLUTION OF A CRYSTALLIZABLE ORGANIC SOLID TAKEN FROM THE GROUP CONSISTING OF HALOGENATED ALIPHATIC HYDROCARBONS, 1,4-BUTANEDIOL, AROMATIC HYDROCARBONS, HALOGENATED AROMTIC HYDROCARBONS, AND NITRATED AROMATIC HYDROCARBONS, AND, SAID FLUORINE-CONTAINING HALOGENATED ALKANE, SAID FLUORINE-CONTAINING HALOGENATED ALKANE BEING PRESENT IN SAID MUTUAL SOLUTION IN THE AMOUNT OF ABOUT 0.5 TO ABOUT 10 MOLE PERCENT, SAID CRYSTALLIZABLE ORGANIC SOLID BEING MUTUALLY SOLUBLE WITH AND CHEMICALLY INERT TO SAID GASEOUS FLUORINATED ALKANE, AND, SAID ORGANIC SOLID BEING CRYSTALLIZABLE OUT OF SAID MUTUAL SOLUTION. 